1. Field of the Invention
The invention relates to a method for pretreating feedstocks for coal hydrogenation with a preheated hydrogen containing hydrogenation gas.
2. Discussion of the Background
It is known in coal hydrogenation processes that the total required quantity of hydrogenation gas is heated with the coal-oil slurry. It is also known that a part of the hydrogenation gas is preheated separately and added to the coal-oil slurry before the preheater and another part of the hydrogenation gas is preheated separately and added downstream from the preheater to the coal-oil slurry, prior to entering the hydrogenation reactor (cf. EP-OS No. 0 083 830).
The hydrogen-containing hydrogenation gas is made up of two parts; a first part is the circulating gas remaining after separation of the coal liquefaction products and a second part is fresh hydrogen added to make up for hydrogen consumption (cf. "Die katalytische Druckhydrierung von Kohlen, Teeren und Mineraloelen" (The catalytic hydrogenation under pressure of coals, tars and mineral oils), Springer-Verlag Berlin/Gottingen/Heidelberg 1950, p. 36).
It is also known that the coal-oil slurry of finely ground coal and the slurry oil, which is a recycle distillate stream from the operation of a coal liquefaction process, undergoes a swelling stage during heating. Depending on the type of coal, the type of slurry oil and the pretreatment, the swelling of these mixtures takes place within a temperature range of about 280.degree. to 390.degree. C. Appropriate means in which the swelling of the slurry takes place can be provided for upstream from the preheater, said means being an expanded pipe or a bottle-shaped receptacle (cf. DRP No. 715 988). The heat transfer in the vertical heat exchanger pipes of the preheater, on the outside of which flows the heating gas longitudinally and on the inside of which flows the slurry, is a critical step in all coal liquefaction processes.
When heating the three-phase mixture of the coaloil slurry in the presence of the hydrogen-containing hydrogenation gas there is a risk of sedimentation of the solid components in the heat exchanger pipers of the preheater. In addition, the suspension can evaporate to dryness through the evaporation of the low-boiling components of the slurry oil.
As a result of the swelling process, a great increase in viscosity occurs in the section between the initial heat exchanger(s) for the slurry and the hydrogenation gas mixture and the preheater. The increase in viscosity can cause a considerable pressure drop in the absence of special precautions. This loss of pressure would have to be compensated by conveying means as pumps and the like.